How does a coil work?

Garry Lovett from Snap-on examines modern coil-pack ignitions

Published:  27 September, 2013

By Garry Lovett

As you all know, the four core checks on a non-starting vehicle are fuel, spark, compression and everything in the right order. Checking for a good spark on a modern vehicle is becoming more challenging with the introduction of coil packs. Many manufacturers have moved over to the coil on plug designs so we no longer have easy access to the coil secondary output, a common test for a spark, meaning we need to assess the system differently.

Some systems have the coil driver inside the coil and the ECU controls the ignition primary system by providing a Pulse Width Modulation (PWM) signal to control the coil driver. In this instance, the primary voltage will not mirror secondary voltage; it will just be an on-off pulse. In addition, some coil on plug systems fire two coils together (wasted spark system), where the individual coils are wired in series, such as on Renaults. Here, assessment of the primary winding can be a problem, as you will be seeing two coils firing on one ECU trigger signal: One coil fires on power and the other is wasted on exhaust. The effect is a difference in peak kilovolts (KV) and burn time, making the waveform on the oscilloscope look unstable.

An amps clamp would normally be put around the live feed to the coil and connected to the oscilloscope. Using this method with a suitable time-base you can see all coils firing. This is useful, as it will be easy to compare the current on each coil when looking for a problem. Current flowing will always increase or 'ramp' at an angle and this can be used for testing purposes, as many systems have specifications for rise time and peak current. For example, a one amp increase per millisecond with a peak current of seven amps.

A slightly different approach is to use the magnetic field around an ignition coil - an aerial or flag is placed directly on the coil and it is used to pick up the changes in electromagnetic field. This induced voltage can be used to light up a light emitting diode (LED). Although quick and easy, the LED is not capable of giving important measurement points like Peak KV, Spark KV and burn time, which can lead to an incorrect diagnosis. A better solution is to use these flags or pickups with an oscilloscope which is capable of giving these important measurements.

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